1. Field of the Invention
The present invention relates in general to vehicle headlamps, and more particularly to vehicle headlamps of a so-called "projector type" which comprises an electric bulb for generating light, a concave reflector for reflecting the light forward, a shade plate placed in front of the concave reflector for partially shading and thus contouring the reflected light, and a coverging lens placed in front of the shade plate for projecting the contoured light beam forward.
2. Description of the Prior Art
In order to clarify the task of the present invention, a conventional projector type conventional vehicle headlamps will be described with reference to FIGS. 13 to 15.
FIGS. 13 and 14 show schematically the conventional headlamp which is generally designated by reference "a". This headlamp "a" is used as a lower beam projector which projects the light beam intended for a forward neighbouring illumination particularly used when the vehicle is meeting or following other vehicles.
Designated by "b" is a concave light reflector which has an ellipsoidal light reflecting surface "c" formed on an inside face thereof. A light source "d" is placed on a first focus "f.sub.1 " of the light reflecting surface "c". Thus, light rays emitted from the light source "d" are reflected forward by the reflecting surface "c" and converged at a second focus "f.sub.2 " of the surface "c". Because the light source "d" has certain dimensions, the bundle of the light rays converged at the second focus "f.sub.2 " has a certain sectional area.
Designated by reference "e" is a shade plate whose upper edge is designated by reference "f". The shade plate "e" is arranged in front of the light reflector "b" in such a manner that the upper edge "f" thereof is located in the vicinity of the second focus "f.sub.2 " of the reflecting surface "c". The line designated by "X--X" is the optical axis of the light reflecting surface "c".
A converging lens "g" is arranged in front of the shade plate "e" in such a manner that a focus "f.sub.c " is placed on a center of the upper edge "f" of the shade plate "e".
When, with the arrangement as described hereinabove, the light source "d" is energized, the light rays emitted therefrom are reflected forward by the light reflector "b" and converged at the second focus "f.sub.2 " of the reflector "b". Due to presence of the upper edge "f" of the shade plate "e" near the second focus "f.sub.2 ", part of the reflected light rays from the reflector "b" is shaded. Thus, the light beam projected forward from the lens "g" has such a contoured cross-sectional pattern "b" as shown in FIG. 15. That is, the projected beam from the lens "g" has an inverted image of the upper edge "f" of the shade plate "e". The bent line "i" in the pattern "h" in FIG. 15 is provided by the upper edge "f" of the shade plate "e". It is to be noted that, in FIG. 15, the line denoted by "H--H" is a horizontal line extending perpendicular to the optical axis "X--X", while the line denoted by "V--V" is a vertical line extending perpendicular to the optical axis "X--X".
However, due to its inherent construction, the above-mentioned conventional headlamp has the following drawbacks.
That is, the projected beam pattern "h" produced by the headlamp is poor in producing emphasized illuminance and in obtaining a desired illumination expansion.
In fact, the illuminance of the projected beam pattern "h" is gradually lowered as the distance from the center of the pattern "h" increases. This is because the luminous flux at the upper edge "f" of the shade plate "e" is gradually reduced with increase of a distance therefrom in the radial direction. Furthermore, when the lateral size of the projected beam pattern "h" is increased, the illuminance of the center zone of the same is inevitably lowered. These phenomena do not meet the requirement of the vehicle headlamps.